1. Field of the Invention
The present invention relates to a sheet supply apparatus used with an image forming apparatus such as a copying machine, a printer, a facsimile machine and the like.
2. Related Background Art
In the past, generally, in sheet supply apparatuses used with an image forming apparatus such as a copying machine, a printer, a facsimile machine and the like, sheets stacked on a sheet stacking tray were conveyed one by one by rotational friction of a sheet supply roller. In the sheet supply apparatus of this kind, a surface of the sheet supply roller is formed from elastic material such as rubber, so that the sheet supplying ability greatly depends upon the coefficient of friction of the roller surface. Accordingly, the sheet supplying ability was unstable due to the change in the outer configuration of the sheet supply roller because of the frictional wear, the change in the material of the sheet supply roller through the elapsed time and the change in the coefficient of friction of the roller surface because of the adhesion of paper powder, and other sheets having different surface conditions could not be handled.
To avoid this, recently, there has been proposed a sheet supply apparatus (referred to as "air sheet supply apparatus" hereinafter) in which sheets stacked on a sheet stacking tray is absorbed and conveyed by utilizing an absorbing force of air and a conveying force of an endless belt. FIG. 49 shows a typical example of such air sheet supply apparatus. A sheet convey portion 100 is arranged above a sheet stacking tray 101 on which a plurality of sheets S are stacked. The sheet convey portion 100 comprises a sheet absorb means 102, a convey belt 103 having a plurality of air openings 103A, and a blower 104 for absorbing or sucking air below the convey belt 103 through the air openings 103A and an opening 102A of the sheet absorb means 102.
A nozzle 105 for injecting air to float an uppermost sheet S and a nozzle 106 for injecting air against second, third and other sheets S to separate the uppermost sheet S from the other sheets are arranged in the proximity of a tip end of the sheet stack S rested on the sheet stacking tray 101. The nozzles 105, 106 are connected to a blower 107 so that the air is supplied to these nozzles.
In this air sheet supply apparatus, in order to supply the sheets S stacked on the sheet stacking tray 101, first of all, the air is injected from the nozzle 105 by the action of the blower 107 to float several sheets S on the sheet stacking tray 101. Then, by the absorbing force of the sheet absorb means 102 generated by the action of the blower 104, the uppermost sheet S is absorbed to a sheet absorb surface 103B of the convey belt 103. Thereafter, the convey belt 103 is rotated in a direction shown by the arrow to convey the sheet absorbed to the convey belt 103 in a downstream direction. In this case, the air from the nozzle 106 is blown against a tip end of the sheet S by the action of the blower 107, thereby separating the double-fed second and other sheets (if any) from the uppermost sheet S. In this way, only the uppermost sheet S is sent to a pair of convey rollers 108.
Incidentally, in this air sheet supply apparatus, the sheet S floated by the air from the nozzle 105 and absorbed to the sheet absorb surface 103B of the convey belt 103 by the absorbing force of the sheet absorb means 102 is absorbed to the sheet absorb surface 103B of the convey belt 103 in a condition that the sheet is substantially parallel with the convey belt through the whole length of the sheet. Further, the endless convey belt 103 extends between and wound around two rollers 109, 110 arranged on both sides of the sheet absorb means 102. In this example, the downstream roller 109 is a drive roller rotated by a motor 111, and the upstream roller 110 is a driven roller.
Incidentally, the air generated by the blower 107 flows toward the nozzle 105 when a valve 112 is switched to a position a, and flows toward the nozzle 106 when the valve 112 is switched to a position b. Further, the sheet absorb means 102 is so designed that the absorbing force is generated by the action of the blower 104 when a valve 113 is in a position c, and the absorbing force is not generated when the valve 113 is in a position d.
By the way, in the above-mentioned air sheet supply apparatus, an important factor for separating the sheets S is to surely blow the air from the nozzle 106 between the first sheet S1 to be conveyed and the second sheet S2 to be separated. As shown in FIG. 50, when the air from the nozzle 106 is blown between the first sheet S1 and the second sheet S2, the positive pressure is generated between the sheets, with the result that a downwardly directing force for aiding the separation acts on the second sheet S2 and an upwardly directing force for aiding the absorption of the sheet to the convey belt 103 acts on the first sheet S1. Thus, the first sheet S1 is separated from the second sheet S2. In this way, when any gap exists between the tip end of the first sheet S1 to be conveyed and the tip end of the second sheet S2 to be separated, the air from the nozzle 106 can enter between the sheets S1, S2 to separate the second sheet S2 from the first sheet S1.
However, in the above-mentioned conventional air sheet supply apparatus in which the sheet S floated by the air from the nozzle 105 and absorbed to the sheet absorb surface 103B of the convey belt 103 by the absorbing force of the sheet absorb means 102 is absorbed to the sheet absorb surface 103B of the convey belt 103 in the condition that the sheet is substantially parallel with the convey roller through the whole length of the sheet, the second sheet S2 sometimes could not be separated from the first sheet S1.
That is to say, as shown in FIG. 51, if the tip end of the first sheet S1 is closely contacted with the tip end of the second sheet S2 due to the sticking between fibers of first and second sheets and/or the flash of the cut edges of the sheets generated by the poor cutting, the air from the nozzle 106 will not enter between the first sheet S1 and the second sheet S2, with the result that the positive pressure is generated below the second sheet S2. Accordingly, in this case, the air acts on the undersurface of the second sheet S2 to promote the close contact between the first sheet S1 and the second sheet S2. In particular, in thin sheets, it is feared that the second sheet S2 is absorbed to the convey belt 103 together with the first sheet S1 by the absorbing force of the air from the air openings 103A of the convey belt 103.
In this way, when the tip end of the first sheet is closely contacted with the tip end of the second sheet, it is impossible to separate the second sheet S2 from the first sheet S1. As a result, there arose a problem that the double-feed of the sheets could not be prevented. Further, in order to prevent the double-feed, if the force of the air injected from the nozzle 106 becomes stronger, the first sheet S1 to be conveyed will also be blown out.